Automatic transmissions mounted to vehicles are roughly divided into two types, geared and gearless. The geared automatic transmission is formed of a fluid coupling such as a torque converter and a gear transmission mechanism.
The geared automatic transmission is connected to an engine via the fluid coupling such as a torque converter. The geared automatic transmission is formed of a transmission mechanism (planetary gear deceleration mechanism) having a plurality of power transmission paths, and is configured to automatically switch the power transmission paths in accordance with accelerator opening and vehicle speed, for example, or to automatically switch the gear ratios (forward driving position). In the geared automatic transmission, clutch, brake and one-way clutch elements as friction elements are engaged or disengaged (released) as prescribed, to determine the gear levels.
A vehicle having such an automatic transmission is usually provided with a shift lever operated by a driver. Operating the shift lever can set a shift position (e.g., reverse drive position, neutral position, or forward drive position).
When the vehicle having the automatic transmission configured as described above is let out of a garage before driving or put into a garage after driving, a so-called garage shift is effected, from the parking (P) position to the reverse drive (R) position for moving backward, or from the neutral (N) position to the forward drive (D) position or the reverse drive (R) position to start driving.
The garage shift includes the case where the state of the automatic transmission is changed from a non-drive position (neutral (N) position) to a drive position (forward drive (D) position). Techniques to solve various problems arising in such a case have conventionally been disclosed.
For example, Japanese Patent Laying-Open No. 2-190660 discloses a shift control device for an automatic transmission that compensates for oil pressure necessary to surely fasten a friction engagement element at the time of stall start, while preventing an excessive shock at the time of racing select start, upon shift from the neutral (N) position to the forward drive (D) position with an engine rotating at high speed. The shift control device of the automatic transmission includes range transfer detecting means for detecting a select from a power cutoff range to a forward or reverse drive range, rotating speed detecting means for detecting the rotating speed of the engine at the power cutoff range, rotating state determining means for determining, when the range transfer detecting means detects a select signal from the power cutoff range to the forward or reverse drive range, whether the engine rotating speed detected by the rotating speed detecting means is a preset value or above indicating the racing select start, and liquid pressure lowering means for lowering the fastening pressure of the friction element than a value set in a normal control when the rotating state determining means determines that the engine rotating speed is the preset value or above.
With this control shift device of the automatic transmission, when the select to the drive range is detected by the range transfer detecting means, the fastening pressure is decreased by the liquid pressure lowering means only if the engine rotating speed at the power cutoff range detected by the rotating speed detecting means is a preset value or above. Thus, it is possible to mitigate the excessive fastening shock by lowering the fastening pressure at the time of racing select start when the engine rotating speed is increased in the power cutoff range for start. On the other hand, at the time of normal stall start when the engine rotating speed is increased after the range is switched to the running range, the engine rotating speed detected by the rotating speed detecting means is low, with which the liquid pressure lowering means is not operated. As such, the ordinary high fastening pressure is secured, thereby ensuring fastening of the friction engagement element.
With the shift control device of an automatic transmission disclosed in Japanese Patent Laying-Open No. 2-190660, however, only the fastening pressure of the friction element is decreased from a value set in the normal control when the rotating state determining means determines that the engine rotating speed is a preset value or above. As such, the friction engagement element is engaged while the engine rotating speed is high, although the engagement pressure is lowered with the liquid pressure lowering means.
That is, the friction engagement member should be engaged while the input revolution number to the automatic transmission is high, which requires enhancement in strength, heat resistance and the like of the friction engagement element. As a result, the friction engagement element suffers the following problems:
1) cost increase (increase in number of the friction member sets, increase in thickness of the separator plates);
2) weight increase (increase in number of the friction member sets, change in quality of the friction members, increase in thickness of the separator plates); and
3) occurrence of shift shock (increase of torque gain with respect to oil pressure due to increase in number of the friction member sets, increase of peak torque due to increase of engagement oil pressure and reduction of gear change time).